Observation of joint structures with ultrasound, though described in many articles in the past decades, has failed to become an accepted joint measurement tool. As early as 1983, ultrasound was suggested as a means of observing joint cartilage. As an example, Alex M. Aisen, MD et. al. Radiology Vol. 153 Number 3, December 1983, “Sonographic Evaluation of the Cartilage of the Knee,” demonstrates the use of ultrasound in observing “changes in the surface characteristics and thickness of cartilage”. This ultrasound method relies on subjectively evaluating an ultrasound image.
Ultrasound has the potential to be useful in observing some joint structures besides cartilage. According to Iagnocco, et al.Scand. J Rheumatol Vol. 21: 201–203, 1992 in “Sonographic Evaluation of Femoral condylar Cartilage in osteoarthritis and rheumatoid arthritis,” “Sonography of the knee makes it possible to study the anatomical details of this articulation such as synovial membrane, intra-articular fluid, articular cartilage, ligaments and tendons, menisci, possible popliteal cysts, their dimensions and location.” Again, this ultrasound method relies on subjective evaluation of an ultrasound image.
Allessandro Castriota-Scanderbeg et al., in “Skeletal age assessment in children and young adults: Comparison between a newly developed sonographic method and conventional methods,” Skeletal Radiology 1998 27:271–277, propose a method for assessing skeletal age using ultrasound imaging measurements of the thickness of femoral head articular cartilage.
Ultrasound can also provide important information about horse joint conditions that is important to their trainers, as stated by Tomlinson BVSc., et. al., Veterinary Radiology and Ultrasound Vol. 41 no. 5, 2000 pp. 457–460 “Ultrasonographic evaluation of Tarsocrural Joint Cartilage in Normal Adult Horses,” “Diagnostic ultrasound has proven to be more sensitive than radiology for the early identification of periarticular remodeling and osteophyte formation” in evaluating equine Osteoarthritis. Again, this method relies on subjective evaluation of an overall ultrasound image.
Ultrasound observation of joint structures is advantageous because its probes can be applied external to the body so it can be used in a non-sterile environment. Ultrasound provides joint inspection without using ionizing radiation as does X-ray, allowing examinations to be repeated frequently without risk to the patient. Ultrasound does not require a special media, such as an X-ray plate, with which to gather data and can provide better resolution than X-ray, particularly of the cartilage and soft tissue structures. Also, ultrasound is a less expensive imaging unit than X-ray, CT or MRI. For these reasons, ultrasound is useful for imaging in a doctor's office, mobile clinics, theater of battle or in screening programs in undeveloped countries.
Yet, for all its advantages, ultrasound has not lived up to its potential. As noted by Walter Grassi et al., June 1999 Seminars in Arthritis and Rheumatism vol. 29, No. 6. “Sonographic Imaging of Normal and Osteoarthritic Cartilage,” as of mid-1999, ultrasound potential “is still under investigation.”
Ultrasound has apparently not gained widespread use for assessing joints beyond imaging, in spite of its apparent suitability.
Overview of Joint Structures
Joints are connections, typically movable, between two or more bones and include many structures that lend themselves to quantitative measurement. Such structures include, but are not limited to cartilage, chondrocytes, subchondral bone, joint capsule, joint fluid, ligaments and tendons:
(a) Cartilage is a tissue that consists of cells called chondrocytes in an extracellular matrix. The cartilage coats the ends of bones within a joint. Cartilage acts as a shock absorber against impact to the ends of bone during actions such as walking, running or jumping. The surface of cartilage provides an almost friction-free surface between bones as they move against each other.
(b) Chondrocytes are cells that lay down new cartilage. Chondrocytes produce and maintain the extracellular matrix.
(c) Subchondral bone, is a part of the bone that is located directly below cartilage and demonstrates changes in a variety of states that affect a joint.
(d) Joint fluid is a complex liquid that serves to nourish the cartilage, lubricate the tissue of a joint and carry waste away from cartilage.
(e) Joint capsule is a flexible tissue joint covering that gives a joint support and seals in the joint fluid that lubricates the joint structures. Its inner lining is the synovial membrane that produces and replenishes joint fluid.
(f) Ligaments are fibrous bands that connect one bone to another, usually within or part of a joint capsule.
(g) Tendons are the fibrous ends of a muscle that attaches it to bone and can be incorporated into joint structures.